When you defer money into your 401k plan, your employer withholds the money from your paycheck and puts it into your 401k plan. At the end of the year, when your employer prints your W-2 showing your taxable income for the year, your 401k plan contributions are not included, so that the money is never reported on your income taxes. Therefore, you cannot claim an additional deduction. If you know your marginal income tax rate, you can calculate how much your 401k plan contributions save you on your income taxes.

Find your marginal income tax bracket using the tax rate schedule for your filing status in IRS Publication 17. Using your income tax filing status rate schedule, find the bracket that includes your total taxable income. For example, if in 2010 you were married filing separately and your total taxable income equals $80,000, you fall in the 28 percent income tax bracket.

Divide your marginal income tax rate by 100 to change it to a decimal. In this example, divide 28 percent by 100 to get 0.28.

Multiply your marginal tax rate written as a decimal by 100 to calculate your tax savings from your 401k plan contributions. In this example, if you contributed $8,800 to your 401k plan, multiply $8,800 by 0.28 to find your tax savings equal $2,464.